Production of iron powder having high electrical resistivity



United States Patent Ofifice 3,245,841 Patented Apr. 12, 1966 3,245,841 PRODUCTION OF IRON POWDER HAVING HIGH ELECTRICAL RESISTIVITY Sydney George Clarke, 44 Holland Way, Hayes, Kent, England, and Jack Frederick Andrew, 25 Chapel Farm Road, London, England No Drawing. Filed Aug. 15, 1962, Ser. No. 216,986 Claims priority, application Great Britain, Aug. 31, 1961, 31,348/ 61 Claims. (Cl. 1486.16)

The present invention relates to the production of iron powder having high electrical resistivity.

Theinvention provides iron powder ofhigh specific resistivity in excess of one megohm/cm. cube which retains its electromagnetic properties and is suitable for use as a magnetic core in a wide range of electrical components, including high frequency electrical components, such as R.F. chokes and transformers, as compacts either of the iron powder itself or more usually of the iron powder with a binder. A particular use is in an attenuator element capable of attenuating electromagnetic Waves having, for example, wavelengths of the order of one cm.

In accordance with the invention, a process for making highly resistant iron powder comprises treating iron powder with phosphoric acid and chromic acid in solution until the iron particles are provided with a resistant surface coating consisting principally of phosphates of iron and chromium compounds whereby the iron powder when formed into a compacted mass has a specific resistivity of at least one megohrn/ cm. cube. The presence of chromic acid assists in the formation of a stable and largely continuous iron phosphate coating and by limiting the amount of phosphoric acid used (provided there is at least about /2% by weight of the weight of iron powder to be treated) a very thin yet resistant coating may be formed which leaves the treated particles pre dominantly as metallic iron yet having a very high electrical resistivity in a compacted mass.

The treatment is readily carried out by suspending the iron particles in a dilute solution containing phosphoric acid and chromic acid. The chromic acid need not be present initially if its addition later is found to be more satisfactory when relatively unreactive iron powders are treated.

The weights of phosphoric acid and [chromic acid (chromium trioxide C O employed are each preferably /2 to 2 /z% respectively of the weight of iron powder. The amount 'of resistant coating produced is dependent upon the amount and concentration of acids employed which are therefore normally limited to that required to produce a powder having a desired value of specific resistivity.

The volume of solution should be sufficient for the iron powder to be readily suspended by rapid stirring (the weight of water being two or more times the weight of powder), and to give a suitable degree of dilution for the reaction with the phosphoric and chromic acids. In order to prevent the acidity from being too high while the coating is being formed, the acids may advantageously be added gradually. A suitable concentration of each acid during this period is about 0.1% or less, preferably about 0.05% or less, with the action finishing through exhaustion of the acids.

The process may be aided by raising the solution temperature, conveniently to say 5080 C. It should be carried out as free as possible from contaminants using materials of high purity.

An iron powder which can be treated to have a very high specific resistance (exceeding 100 megohms/cm. cube) is composed of substantially spherical particles 1-10 microns in size, such as carbonyl iron powder produced from iron carbonyl.

The treatment of a 2 kg. batch of iron powder is carried out on the following general lines.

The iron powder is vigorously agitated in about four litres of water at 5080 C. One litre of a solution containing the order of 20 grams of chromic acid and 20 grams of phosphoric acid is slowly added over a period of 10-30 minutes to the agitated liquid held at 5080 C. The powder is finally separated from .the liquid, thoroughly washed and dried.

Thicker insulating coatings are normally produced by correspondingly increasing both the amounts of the acids employed and the duration of the treatment.

The response of the iron powder to surface coating by this treatment depends upon the purity of the powder, in particular, for carbonyl iron powders on its carbon content. Low carbon is generally less reactive than high carbon iron and the following specific process for a low carbon carbonyl iron containing less than 0.05% C.-is given by way of example.

200 grams of the iron powder are added to 400 ml. of distilled water at 65 70 C. with vigorous stirring. 100 ml. of a solution containing 1.3 ml. of phosphoric acid (sp. gr. 1.75) is then added to the stirred suspension, 10 ml. at a time, at one minute intervals, 100 ml. of a solution containing 2 grams of chromic acid is then added, 10 ml. at a time, similarly at one minute intervals. Stirring is continued for a further five minutes and the temperature of the mixture is maintained throughout at 65 -70 C.

500 ml. of distilled water are then added with thorough stirring. The powder is then allowed to settle and the liquid decanted off. The washing and liquid separation is repeated two or three times until the decanted liquor is free from yellow coloration and the soluble chemicals have been washed away.

The powder is then stirred in 500 ml. of methyl alcohol, filtered off (using a Biichner funnel and Whatman filter paper No. 541) rewashed with 200 ml. of methyl alcohol and finally spread out and dried in an oven at about C. for one hour.

If the iron powder has a higher carbon content, e.g. 0.6% C the process may be simplified. Instead of adding a phosphoric acid solution followed by a chromic acid solution, 100 ml. of a combined solution containing both 1.3 ml. of phosphoric acid (sp. gr. 1.75) and 2 grams of chromic acid is added 10 ml. at a time but at two minute intervals so that the solution addition time is still 20 minutes.

As a result of this treatment high carbon powder becomes a dark khaki colour but low carbon powders are dark grey and only slightly changed in colour.

Well treated carbonyl iron powders (average particle size 3-7 microns) of both high and low carbon content should still retain a high proportion of its metallic iron content (over while containing a sufficient coating on the particles to have an overall phosphate content of 0.71.0% and a chromium content of 0.1O.5%. The thickness of the coating is probably of the order of 0.05 microns.

The most important characteristic of treated carbonyl iron powders is the high value of electrical resistance. For compacted powders of both high and low carbon content the measured specific resistivity is over one hundred megohm/cm. cube.

We claim:

1. A process for making a highly resistant iron powder comprising treating iron powder whose particle size is up to 10 microns, with /2 to 2 /z% of its weight of phosphoric acid and /2 to 2 /2% of its weight of chromic acid dissolved in at least two liters of water for each kilogram al of iron powder with suflicient agitation to keep the particles in suspension, until the iron particles are provided With a resistant surface coating, whereby the iron powder when formed into a compact mass has a specific resistivity of at least 1 megohm/cm. cube.

2. A process according to claim 1 carried out at between 50 and 80 C.

3. A process according to claim 1 in which the phosphoric acid and the chromic acid are added over a period of time so that the concentration of each acid in the solution does not exceed about 0.1% at any time.

4. A process for making highly resistant iron powder comprising the steps of suspending low carbon carbonyl iron powder whose particle size is up to microns in at least two liters of water for each kilogram of iron powder, treating the suspended iron powder initially with an aqueous solution containing /2 to 2 /2% by weight of phosphoric acid based on the quantity of the iron powder, adding afterwards an aqueous solution containing /2 to 2%% by weight of chromic acid with reference to the iron powder, and agitating sufiiciently to keep the particles in suspension throughout the treatment, until the iron particles are provided with a highly resistant surface coating whereby the iron powder when formed into a compacted mass has a specific resistivity of at least 100 megohm/ cm. cube.

5. A process according to claim 4 which is carried out 50-80 C.

6. A process according to claim 4 in which the phosphoric acid and the chromic acid are added over a period of time so that the concentration of each acid in the solution does not exceed about 0.1% at any time.

7. A process for making highly resistant iron powder comprising the steps of suspending high carbon carbonyl iron powder whose particle size is up to 10 microns in at least two liters of aqueous solution for each kilogram of iron powder, treating the iron powder with a mixture in aqueous solution of /2 to 2 /2 by weight each of phosphoric and chromic acid with reference to the iron "ly to keep the particles in suspension, until the iron particles are provided with a highly resistant surface coating whereby the iron powder when formed into a compacted mass has a specific resistivity of at least 100 megohm/cm. cube.

8. A process according to claim 7 carried out at -80 C.

9. A process according to claim 7 in which the said acid mixture is added over a period of time so that the concentration of each acid in the solution does not exceed about 0.1% at any time.

10. Highly electrically resistant iron powder, having a particle size up to 10 microns, and which has an iron content of over and'a coating of the order of 0.05 micron thick, which coating consists essentially of iron phosphates and chromium compounds obtained by treatment of the iron powder with dilute aqueous phosphoric and chromic acid and having, when formed into a compacted mass, at specific resistivity of at least rnegohm/ cm. tube.

References Cited by the Examiner UNITED STATES PATENTS 1,850,181 3/1932 Roseby. 1,982,690 12/ 1934 Polydoroif. 2,306,198 12/1942 Verweij ert al. 1486.l5 XR 2,403,426 7/1946 Douty et al. 148-616 2,516,685 7/1950 Douty et al. 1486.16 2,634,225 4/1953 Benzing 1486.16 2,783,208 2/1957 Katz 1486.15 XR FOREIGN PATENTS 837,210 2/ 1939 France.

JOSEPH B. SPENCER, Primary Examiner WILLIAM D. MARTIN, RICHARD D. NEVIUS,

Examiners. 

1. A PROCESS FOR MAKING A HIGHLY RESISTANT IRON POWDER COMPRISING TREATING IRON POWDER WHOSE PARTICLES SIZE IS UP TO 10 MICRONS, WITH 1/2 TO 2 1/2% OF ITS WEIGHT OF PHOSPHORIC ACID AND 1/2 TO 2 1/2% OF ITS WEIGHT OF CHROMIC ACID DISSOLVED IN AT LEAST TWO LITERS OF WATER OF EACH KILOGRAM OF IRON POWDER WITH SUFFICIENT AGITATION TO KEEP THE PARTICLES IN SUSPENSION, UNTIL THE IRON PARTICLES ARE PROVIDED WITH A RESISTANT SURFACE COATING, WHEREBY THE IRON POWDER WHEN FORMED INTO A COMPACT MASS HAS A SPECIFIC RESISTIVITY OF AT LEAST 1 MEGOHM/CM. CUBE. 